Many image display systems generate display images and project them onto display screens, typically for viewing by multiple persons or viewers. The display images may be formed by transmitting light from a high-intensity light source through an image-forming medium such as a translucent film or liquid crystal display (LCD). Alternatively, the display images may be formed by reflecting the light off of a reflective image-forming medium such as a digital micro-mirror display. The following description is directed by way of example to transmissive LCD display systems, but is similarly applicable to reflective display systems.
FIG. 1 is a schematic side view optical diagram of a conventional prior art front LCD projector 10 that projects display images along a central optic axis 12 upward toward a reflective display screen 14. LCD projector 10 includes an illumination or light source 16 and a light-collecting reflector 18 that direct polychromatic, substantially white light through a spherical condenser lens 20. A display system configured as a front LCD projector is positioned with the viewers in front of a reflective display screen. A display system configured as a rear projection display is positioned opposite the viewers behind a transmissive display screen.
A fold mirror 21 directs the light through a Fresnel collimating lens 22 that collimates the light for delivery to an image source 24 such as a transmissive color thin-film transistor (TFT) active matrix LCD together with any required polarizing elements. As is known in the art, image source 24 controls or modulates the substantially white light to form a multi- or full-color display object according to a video display signal delivered to image source 24 from display control circuitry (not shown). The light that passes through the display object propagates to a Fresnel converging lens 26 that focuses the light toward a conventional objective projection lens assembly 28 such as a varifocal lens assembly.
Typically, reflector 18 is of a parabolic or elliptical shape and forms a round illumination pattern. Image source 24 typically has a rectangular configuration. To assure that all of it is illuminated, image source 24 is overfilled by the round illumination area, which causes between 30 and 40 percent of the illumination to be lost or wasted. Illuminating the corners of image source 24 with intensity equal to that of its center causes even more light to be lost.
Illumination efficiency can be improved by using a reflective tunnel integrator or homogenizer having a rectangular output face. The homogenizer may be formed as a hollow mirror tunnel or a solid rectangular glass rod. A benefit of such a homogenizer is that it increases both the uniformity and the efficiency with which image source 24 is illuminated. With conventional optics, uniformity and efficiency are typically opposed characteristics. A disadvantage of such a tunnel homogenizer, however, is that it is very bulky and has a long path length requirement, thereby being ill-suited to portable projection systems.
Attempting to shorten such a long path length with a conventional spherical relay lens system introduces pin cushion distortion into the illumination pattern formed by the tunnel homogenizer. This pin cushion distortion weakens the illumination intensity at the edges of image source 24, particularly at its corners, and also decreases the overall illumination efficiency. As a consequence, attempts to shorten the path length requirements of a tunnel homogenizer have required unacceptable loss of the uniformity and efficiency enhancements of the homogenizer.
Prior art LCD projectors commonly suffer from illumination non-uniformity and excessive losses of illumination. Such losses are particularly problematic because of illumination intensity limitations in transmissive LCD displays. Although reflective tunnel homogenizers are capable of providing improved uniformity and efficiency, the path length and size requirements for such homogenizers are incompatible with LCD projectors and other image display systems.